1. Field of the Invention
This invention relates to a method of producing grain oriented silicon steel sheets having improved bending properties and magnetic properties in the rolling direction of the sheet.
2. Related Art Statement
The grain oriented silicon steel sheets are mainly used as a iron core for transformers and other electrical machinery and apparatus, so that it is required to have excellent magnetic properties, particularly low iron loss (represented by W.sub.17/50).
For this end, in the grain oriented silicon steel sheet, it is required to highly align &lt;001&gt; orientation of secondary recrystallized grains in the steel sheet toward the rolling direction and also to reduce impurities and precipitates existent in steel of the final product as far as possible. In the grain oriented silicon steel sheets produced by considering these requirements, the iron loss value has been improved from year to year by many efforts up to the present. Recently, there are obtained low iron loss products having a thickness of 0.23 mm and indicating a W.sub.17/50 value of about 0.90 W/kg.
However, it strongly tends to proVide electrical machinery and apparatus having less power loss with last energy crisis, and consequently it is demanded to develop grain oriented silicon steel sheets having a lower iron loss value as a material for the iron core.
As a method of reducing the iron loss of the grain oriented silicon steel sheet, there are generally known metallurgical methods, i.e. a method of increasing Si amount, a method of thinning the product thickness, a method of fining secondary recrystallized grains, a method of reducing the amount of impurities, a method of highly aligning secondary recrystallized grains into (110)[001] orientation, and so on.
In order to highly align the secondary recrystallized grains into (110)[001] orientation, it is necessary to rapidly conduct secondary recrystallization while sufficiently controlling the growth of normal grains, so that the reinforcement of control force is required.
As a means for reinforcing the control force, the addition of Cu to steel has been known from the old time. For example, Japanese Patent Application Publication No. 48-17688 discloses that the control force is reinforced by adding 0.10-0.30% of Cu to migrate MnTe into grain boundary. Further, Japanese Patent laid open No. 50-15726 proposes a technique that the restriction of hot rolling conditions due to the precipitation of inhibitor is mitigate by adding 0.1-0.5% of Cu and using manganese copper sulfide as an inhibitor to lower the dissolving temperature of the inhibitor during the heating of slab. And also, Japanese Patent Application Publication No. 54-32412 discloses a technique that the magnetic flux density is increased by adding 0.2-1.0% of Cu or Ni and making proper the draft and the final finish annealing. Moreover, Japanese Patent laid open No. 61-12822 discloses a technique that the control force is reinforced to improve the magnetic properties by adding 0.02-0.20% of Cu to finely precipitate (Cu, Mn).sub.1.8 S as an inhibitor.
According to the inventors' studies, however, it has been found that the addition of Cu to steel is not essential to the reinforcing effect of the control force but is effective to the degradation of the control force at the surface layer portion of the steel sheet. In this connection, the inventors have found that since the control force at the surface layer portion of the steel sheet in the secondary recrystallization is degraded at the annealing step in the factory production, in order to avoid such a degradation phenomenon and maintain the sufficient control force at the surface layer portion, it is effective to uniformly adhere a metal having an electrode potential higher than that of Fe to the steel sheet surface before or after decarburization and primary recrystallization annealing, and disclosed this technique in Japanese Patent laid open No. 61-190020.
Incidentally, according to the inventors' studies, it has been confirmed that when Cu is added to steel, the size and distribution of the inhibitor precipitated at the hot rolling step are certainly fine and the precipitating frequency is high, but the inhibitor is apt to cause Ostwald growth by a heat treatment at high temperature in the post steps (for example, annealing of the hot rolled sheet, intermediate annealing, final finish annealing) and consequently the control force is frequently lowered to bring about the degradation of the magnetic properties. Furthermore, in the steel sheets containing Cu, the surface cracking is apt to be caused in the hot rolling, whereby the surface properties of the final product are degraded, and also the side end face of the coil after the final finish annealing is wavily bent or undesirably folded.
That is, the aforementioned problems have been solved by the above technique described in Japanese Patent laid open No. 61-190020 in order to improve the magnetic properties. However, it has been confirmed from later studies that the following problems are still existent in this technique.
Even in the adoption of the above technique, the stability of the magnetic properties is poor and also the breakage is undesirably caused when the final roduct is subjected to a bending work (which is generally called as bending properties). If the transformer is manufactured by using the product having such poor bending properties, the cracking is caused, for example, in the steel sheet to considerably degrade the performances of the transformer, and in the worst case, the insulating property between the laminated steel sheets is obstructed to cause a serious trouble such as baking of the transformer or the like.
In order to avoid these problems, it is effective to select Cu as a metal element to be adhered to the steel sheet surface and increase the amount of Cu adhered as disclosed in the above Japanese Patent laid open No. 61-190020. However, when the amount of Cu adhered is increased, the magnetic properties are largely degraded.